This invention relates to rotary drill bits and more particularly to rock drill bits with a polycrystalline abrasive as the cutting or abrading material.
Conventional rotary drill bits for oil and gas well drilling core drilling have heretofore used cutting elements such as (1) steel teeth, (2) steel teeth laminated with tungsten carbide, (3) a compact insert of sintered tungsten carbide, and (4) natural diamonds all of which are set or molded in a tungsten carbide crown or cone. Due to the relatively short life and/or high operating cost of these conventional designs, it has recently been proposed to use synthetic diamond compacts as the cutting element in such drills.
To date, attempts to use diamond compacts in these applications have, for the most part, been unsuccessful. In one such attempt diamond compacts are comprised of right circular cylinders with a thin layer of polycrystalline diamond bonded to a cemented carbide substrate. A cutting element is formed by attaching the compact to the drill bit by brazing or soldering the carbide substrate to a cemented carbide pin which is inserted into holes in the drill crown. The diamond layer is generally oriented in a radial sense to the center of rotation of the drill bit and penetrates the rock essentially as a cutting tool in a similar manner to a cutting tool which is used to cut metal on a lathe.
Several problems have been encountered with this design and a commercially feasible drill bit has yet to be tested based on this structure.
One problem is that, although in this design the cutting elements protrude from the bit body and thereby provide aggressive cutting action and abundant room for swarf removal, the stresses on each cutting element are severe and frequent failures occur by pin shearing or compact cracking. The stresses are caused because the structure of most rocks is heterogeneous and thus has layers of varying hardness. These layers cause a large variation in the impact loads to be applied to the cutting elements during drilling. The prior art designs are not strong enough, nor are the compacts shock resistant enough, to withstand such widely varying impact loading.
Another problem occurs during manufacturing of the cutting element. The process of brazing the composite compacts to the pin structure requires temperatures approaching those where the diamond layer is degraded. Hence, many of the compacts are "softened" if great care is not taken in the brazing operation.
Still another problem is that the degradation temperature (700.degree. C.) of the compacts is far below the 1200.degree. C. to 1400.degree. C. temperature which would be required to sinter the compacts in an abrasion resistant drill crown matrix (e.g., of tunsten carbide) in an analogous manner to that used to fabricate drill crowns of natural diamond set in the surface of an abrasion resistant matrix.